It is known that high precision control devices frequently have, in addition to a feedback control system, closed loop systems for compensating external noise. These compensation systems use sensors detecting the presence of external noise and generating compensation signals that are added to the control signals used in the feedback loop.
In particular, discrete time control systems are required to synchronize the control signals and the compensation signals so that they may be correctly added.
An application of this compensation technique, to which reference will be made hereinafter, are hard disk read/write devices for controlling the position of reading heads.
As is known, R/W devices, referred to as hard disk drives (HDDs), normally comprise a set of magnetic disks, rotating all together, a head-actuator assembly, and an electronic control unit. The head-actuator assembly comprises a plurality of arms turning about a same rotation axis, integral with one another and actuated by a motor, and reading heads, each reading head being carried by a respective arm and facing respective surfaces of the disks.
Control information is stored in preset control sectors on the disks, is picked up by the heads and enables detection of the position of the heads with respect to the tracks on the magnetic disks. In particular, the heads generate an analog signal that is sampled at a preset rate (depending upon the rotation rate of the disk) to generate a numeric position signal. The electronic control unit detects a position error on the basis of the numeric position signal and generates a numeric control signal for controlling the head-actuator assembly and keeping the heads in optimal reading positions.
In addition, the control system comprises an open loop compensation line for compensating the effects of external disturbance. In particular, an acceleration sensor (for example, a sensor made using MicroElectroMechanicalSystemxe2x80x94MEMSxe2x80x94technology), mounted so as to be integral with the casing of the R/W device, detects any disturbance mechanical vibration and supplies an acceleration signal which is, in turn, sampled at a compensation rate and used by the electronic control unit to generate a numeric compensation signal to be added to the control signal.
The data supplied by the sensor and the control signal are not, however, synchronous, and thus cannot be immediately summed. To overcome this problem, synchronization techniques normally used carry out a sampling rate conversion.
In practice, the sequence of data supplied by the sensor is initially expanded by interposing, between two consecutive samples, a first preset integer number L of null samples corresponding to instants comprised within a same sampling interval. The expanded sequence of data is then filtered using a low pass filter to replace the null samples with interpolated samples. Next, a decimator reduces the expanded sequence of data, maintaining one sample every M samples (where M is a second preset integer number) and eliminating all the others. By selecting the numbers L and M so that the condition       F    c    =                    L        +        1            M        ⁢          F      s      
is satisfied, wherein FC is the control sampling rate and FS is the sensor sampling rate, the sequence of data at the output of the decimator is synchronous with the control signal and can thus be used to generate the compensation signal.
Known devices, however, have a number of drawbacks. In fact, performing frequency conversions increases the phase delay of the compensation signal with respect to the control signal. This is particularly disadvantageous because, as is known to those skilled in the art, the phase delay is a critical parameter for the effectiveness of open loop compensation and must therefore be contained as far as possible. In addition, the frequency conversion is carried out by microprograms (firmware) which, on the one hand, require a physical encumbrance as they must be stored in a nonvolatile memory and, on the other hand, cause an increase in the required processing power.
A further drawback of known devices results from the high frequency noise (approximately 4-5 kHz) normally introduced during the noise measure and the acceleration signal generation. This high frequency noise must be reduced by filtering which, once again, is carried out by microprograms.
The present invention is embodied in a system and method for noise compensation in a discrete time control system. In an exemplary embodiment, the device comprises closed loop control means for generating a first timing signal, a signal indicative of the quantity to be controlled, and a control signal, which have a control frequency. The device further includes open loop control means for generating a compensation signal synchronous with the control signal and supplied to the closed loop control for correcting the control signal. The open loop control means comprises a sensing means for generating an analog signal correlated to a disturbance quantity, sampling means for receiving the analog signal and generating a sampled signal having a sample frequency correlated to the control frequency and a decimator stage for receiving the sample signal generating the compensation signal.
In an exemplary application, the noise compensating device may be used in a read/write (R/W) device, such as a disc drive and comprises a rotatable computer-readable media, a R/W head positioned in proximity with the computer-readable media to read data stored on the computer-readable media and write data to the computer-readable media, and an R/W device control circuit. The control circuit comprises a closed loop control circuit and an open loop control circuit. In one embodiment, the R/W device is a hard disc and the signal indicative of the quantity to be controlled is a track position and the disturbance quantity is a disturbance acceleration.